This invention is directed to a process for reducing the emmissions of environmentally unacceptable compounds from a process for gasifying carbonaceous materials. More particularly, this invention is directed to a process for eliminating or minimizing the amount of ammonia, hydrogen cyanide, and heavy metals, and compounds thereof, heretofore obtained within waste aqueous effluents upon gasifying carbonaceous materials.
Gasification of a carbonaceous material involves reacting it at elevated temperature with air, oxygen, steam, carbon dioxide or various mixtures of such gasifying agents. The typical gasification process produces a gas leaving the gasifier that contains mostly carbon monoxide and hydrogen along with other gases such as carbon dioxide and methane, and distillation products such as tars, as well as carbon and ash. The material entrained with the gas, such as carbon and ash and other particles, are removed usually by a combination of cyclones or centrifugal separators, or a water scrubbing system using washer coolers, venturi scrubbers, or irrigated type electrostatic precipitators or combinations of these systems.
The raw gas from the gasifier contains, in addition to the aforementioned materials, sulfurous gases, such as hydrogen sulfide and carbonyl sulfide, and in addition contains small amounts of ammonia and hydrogen cyanide. Also, the particulates and tars which accompany the gas out of the gasifier may contain compounds of heavy metals such as mercury, zinc, or chromium, which must be properly considered from an environmental standpoint. It is believed that ammonia and hydrogen cyanide are generated predominantly by pyrolysis of nitrogen compounds present in the carbonaceous materials. The heavy metals are present since they are originally present by nature in the carbonaceous material. Since the sulfurous gases, hydrogen cyanide and ammonia, are soluble in water, these compounds are picked up by the water used in the water scrubbers which contact the raw gas in order to remove entrained particulates. As a result, these compounds appear as dissolved gases or chemical compounds in the effluent water from the water scrubbers. Also, the heavy metal compounds may be partially dissolved in ionic form within the effluent water from the water scrubbers.
To accommodate the heat load associated in cleaning the gas, aqueous foul effluent water from the water scrubbers is usually conveyed to a cooling tower where the foul water is cooled by direct evaporation of water to the atmosphere. During this cooling some of the noxious compounds, such as hydrogen sulfide, hydrogen cyanide, and ammonia, that were in the foul effluent water are discharged to the atmosphere. Furthermore, as these gases are stripped from the water in the cooling tower, there is a thermodynamic tendency for even more sulfurous gases, ammonia, and hydrogen cyanide to be absorbed in the water which leaves the cooling tower and is recirculated to the water scrubbers in the gasification process. In addition, dissolved salts from any makeup water that is added to the system and from salts that leach from the carbonaceous fuel or ash will accumulate in the scrubbing water and must be removed by a blowdown stream. The blowdown stream, which is essentially a bleed-off stream, must be treated to remove or render innocuous such compounds as sulfur, ammonia, hydrogen cyanide, as well as heavy metals and salts and compounds thereof that may be contained in the blowdown water.
Methods known in the art for treating aqueous effluents containing ammonia and hydrogen cyanide and compounds thereof are complex and expensive. A common method, known as "breakpoint chlorination," involves purchase of considerable quantities of chlorine or sodium hypochlorite and additional compounds, such as liquid sulfur dioxide, acids and bases. Although such treatments usually permit compliance with current environmental regulations, the treatments produce a treated water that still contains some residual quantities of sulfur, ammonia, hydrogen cyanide and the disolved compounds of the heavy metals. Thus, even the most modern treatment procedures might not achieve an effluent stream which meets the practice encouraged by the U.S. Environmental Protection Agency of maintaining "zero effluents" from process equipment.
It has recently been suggested that the foul effluent water containing the environmentally unacceptable compounds be indirectly cooled to avoid its conveyance to the cooling tower for atmospheric exposure and to thereby eliminate discharge of sulfurous gases, ammonia, and hydrogen cyanide to the atmosphere. The indirect cooling improvement has the inherent disadvantage that an excess of water is produced by condensation of water vapor during the cooling and cleaning operation of the raw gas. This excess water would be contaminated with sulfur, ammonia, and hydrogen cyanide compounds and compounds of heavy metals, and with certain gasification processes, tars may be additionally present. In a gasification process there exists no practical means of removing the excess water from the process. This occurs because steam is heretofore used as a gasifying agent, and, in addition, water is often unavoidably present in the carbonaceous feed, or alternatively in certain instances water may chemically form from oxidation of a portion of the hydrogen present by nature in the carbonaceous feed. It is not practical to separately revaporize the excess water due to the cost and energy intensity and extensive softening that would be required for even a partial revaporization.
Also, in the past various effluents from chemical processes have been disposed of within an incineration process. It would not be environmentally acceptable to dispose of foul effluent water containing ammonia and/or cyanide in this manner. The ammonia and cyanide based compounds, upon incineration, would be largely converted to oxides of nitrogen and these oxides would be released to the atmosphere. These oxides of nitrogen are becoming of increasing concern with respect to adverse environmental impact.
It is the object of the present invention to provide a process for eliminating or reducing from a gasification process the quantities of aqueous effluents containing ammonia, hydrogen cyanide, heavy metals and compounds thereof, plus any sulfur compounds without reducing the efficiency of the process for gasifying carbonaceous materials. It is an additional object of the present invention to provide a process which prevents salts from accumulating in the scrubbing system for raw gas produced in a gasifier and which does not allow foul aqueous effluent from water scrubbers used to clean and to cool the raw gas from a gasifier for carbonaceous materials to contaminate other process waters used in the process for gasifying carbonaceous materials. It is an additional object of the present invention to dispose of the ammonia and cyanide based compounds present in the aqueous effluent from the cleaning and cooling of raw gas produced by the gasification of carbonaceous materials without largely converting the nitrogen contained in such compounds to oxides of nitrogen which would pollute the atmosphere.
It is a further object of the present invention to provide a process for preventing the accumulation of ammonia, hydrogen cyanide, and compounds thereof, and heavy metals and salts and compounds thereof, and sulfur compounds in an aqueous stream which is recirculated to scrub and cool raw gas from a gasifier for carbonaceous materials.